I don't really agree with "this isn't a problem we should solve for them", because that feels like someone claiming to be a friend absolving themselves of actually helping their friends. But I basically see two equally reasonable things they could do.

1. Throw up your hands and say "Gil completely fucked this one, so he gets to reap what he's sowed. What a dipshit." Basically what the Lady Friend is doing, though I would say this is completely giving up the friendship with Gil.
2. Smack Gil in the head, call him a fucking moron, and explain to him how he's spent the last year making his fiancee feel like shit, and if he doesn't completely change his behavior and patch things up with her asap, she's going to dump his ass and become a knight. This is the real friend thing to do imo.